a) Field of the Invention
This invention relates to current collectors for a polymer rechargeable battery and also to a fabrication process of the same. More specifically, the present invention is concerned with current collectors having excellent performance for use in a rechargeable battery and also with a fabrication process of the same. The current collectors according to the present invention are useful in a polymer battery obtained by assembling a cell with a separator made of a polymer and a plasticizer and then replacing the plasticizer with an electrolyte solution.
b) Description of the Related Art
A polymer battery is composed of a positive electrode member and a negative electrode member integrally united together with a polymer electrolyte held as a separator therebetween, and the positive and negative electrode members are provided with current collectors, respectively.
Owing to the use of the polymer electrolyte in place of a liquid electrolyte, the polymer battery can be formed with a reduced thickness and also with excellent safety. Such polymer batteries are hence expected to find utility as batteries for small portable electronic equipments.
Polymer electrolytes can be classified into those consisting solely of polymers and electrolyte salts, respectively, and those of the gel type formed in combination of polymers, electrolyte solutions and electrolyte salts, respectively. The polymer electrolytes of the former type are not fully satisfactory in performance as batteries, so that under the circumstances, gel-type polymer batteries are under development for commercialization.
Gel-type polymer electrolytes, however, are accompanied by problems inter alia in that they are inferior in film-forming properties and they require a special environment upon assembling cells because of the problem of hygroscopicity of lithium electrolyte salts.
To solve the above-described problems, U.S. Pat. No. 5,460,904, which is incorporated herein by reference, discloses to produce a polymer battery by forming a composition, which comprises a polymer and a plasticizer, into a film as a separator, assembling the separator, a positive electrode and a negative electrode into a cell, and then replacing the plasticizer in the polymer with an electrolyte solution. Batteries available from this process are, however, still not considered to be equipped with sufficient service performance.
For elements affecting the performance of the above-described polymer batteries, a study was conducted in various ways. As a result, current collectors were found to give considerable influence. Since a plasticizer is replaced with an electrolyte solution subsequent to the assembly of a cell in such a polymer battery, the current collectors must be in the form of meshes defining therein a number of openings through which a solvent and the electrolyte are allowed to move. Conventionally employed as such current collectors are expanded metals produced by such a method as forming slits in a staggered relationship in metal sheets and then stretching the slitted metal sheets. However, these expanded metals have been found that they are insufficient in strength and may not equipped with tensile strength high enough to permit coating or lamination of a positive or negative electrode material. They have hence been found to be unsuited for line production and also to lead to batteries insufficient in performance, for example, in impedance characteristic, the capacity utilization, and the like.
An object of the present invention is therefore to provide a polymer battery having excellent performance, said polymer battery being produced by forming a composition, which comprises a polymer and a plasticizer, into a film as a separator, assembling the separator, a positive electrode and a negative electrode into a cell and then replacing the plasticizer in the polymer with an electrolyte solution. Another object of the present invention is to provide a current collector extremely suited for use in such a battery.
The above-described objects can be achieved by a current collector for a polymer battery available from assembling a cell with a separator made of a polymer and a plasticizer and then replacing the plasticizer with an electrolyte solution, wherein the collector is available from etching of a metal-foil base material and is provided with a number of openings; and also by a polymer rechargeable battery with such current collectors assembled therein.
The current collector may preferably define therein openings arranged in a honeycomb pattern.
The polymer battery making use of the current collectors according to the present invention, said current collectors having been fabricated by etching, especially the polymer battery making use of the current collectors with openings arranged in a honeycomb pattern are excellent in impedance characteristic and load characteristic. Although reasons for such excellent impedance characteristic and load characteristic have not been fully elucidated, they seem to be attributable to relatively small contact areas and hence, relatively low combined resistances between the respective current collectors and their associated positive electrode film and negative electrode film and also to reduced average current migration lengths within the respective current collectors.